That would depend on the school, the staff, the community and how good you want your school to be. I'd say I do 45-50 in school on average. Plus some evenings. More at busy times ( proofreading 100s of reports takes time!) less at quieter times. I could cut down if I didn't spend as much tune with the children - if I was totally office based I could probably knock 10 hours a week off. But I think I'd be a **** head then, and the school would be very different.